This invention relates to a prefolded shielding tape for shielding electrical conductors including a ground wire and to a cable when formed using such tape.
In the field of telecommunication and data transmission including multiconductor cable assemblies, particular care is required to ensure that adjacent pairs of conductors are isolated from each other and that each pair of insulated conductors is also electrically shielded from its neighbors. If this is not done interference between adjacent pairs of conductors may occur. Furthermore, the shielding of adjacent pairs, throughout the length of the cable, should be isolated so as not to allow random contact between adjacent shields which may cause signal interference. Grounding, or interconnection of shields, is required to be accomplished at designated points only, i.e. at the terminus of the cable, and may be combined with a ground wire which is generally included with each pair of shielded conductors.
Each pair of conductors is helically twisted together during cable manufacture to minimize inductance, and this operation has been combined using one of several methods to apply a shield and insulating dielectric layer usually of aluminum foil and suitable plastics film respectively.
In 1962 U.S. Pat. No. 3,032,604 assigned to Belden Corporation, and now expired, disclosed a method of shielding and isolating a single conductor or pair of conductors by applying a laminate of foil and film with one edge folded back upon itself.
Belden Corporation has promoted the use of a tape in which the opposite edge is folded back upon itself in the opposite direction so that at one edge of the tape the insulating layer is exposed and at the other edge the foil layer is exposed. Such a tape can be wrapped around the conductors and the ground wire in such a way that the foil layer provides a continuous shield and yet is completely covered by the insulating layer so that it cannot contact the shields of adjacent layers.
Such an arrangement is also shown in Canadian Pat. No. 1120562 (Sumitomo).
The manufacture of a cable of this type requires the ground wire to be applied to the twisted conductors either in the twisting process or as a separate step following which the tape is applied as a shield.
This arrangement has a number of disadvantages in that it requires the tape, ground wire and conductors to be applied separately. In addition, the contact between the ground wire and the shield is certainly not guaranteed and can be broken at various points along the length of the cable by the position of the ground wire relative to the conductors.
Canadian Pat. No. 999941 (Boston Insulated Wire and Cable) addresses this problem and provides ground wires in the form of strips embedded between a foil layer and the insulating dielectric layer. In another embodiment the strips are embedded between the foil layer and an additional strip of insulating dielectric layer on the opposite side of the foil layer from the dielectric layer. This arrangement has not achieved any commercial success and has severe disadvantages in relation to its difficulty and cost of manufacture which have probably prevented its effective competition with existing prior art products including those mentioned above.
It is one object of the present invention, therefore, to provide an improved construction of tape which can be used to manufacture a cable of this type which overcomes the above disadvantages.